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= Investigations on update_camera() =
= OpenGL coordinate systems =


In the second part of <math display="inline">\rightarrow</math> axes::properties::update_camera () the view transformation "x_gl_mat1" and projection matrix "x_gl_mat2" are put together. The following images illustrate some of the properties of "x_gl_mat1".
In the Octave plotting backend, we find various OpenGL transformations. Some of the classic OpenGL transformation steps, as well as coordinate systems, are shown in the following picture:


== The Octave coordinate system ==
[[File:Octave_coordinate_systems.png|center|350px]]
 
= The Octave coordinate system =


In Octave a plot scene is defined by a "view point", a "camera target" and an "up vector".
In Octave a plot scene is defined by a "view point", a "camera target" and an "up vector".


[[File:Octave_view_point_setup_to_scale.png|center|750px]]
[[File:Octave_view_point_setup_to_scale.png|center|750px]]
= update_camera() =
In the second part of <math display="inline">\rightarrow</math> "axes::properties::update_camera ()" the view transformation "x_gl_mat1" and projection matrix "x_gl_mat2" are put together. The following chapter illustrates some of the properties of "x_gl_mat1" and "x_gl_mat2".


== The role of "x_gl_mat1" ==
== The role of "x_gl_mat1" ==
=== x_view ===


The following section of code assembles the matrix "x_view", which is a subset of "x_gl_mat1". The matrix "x_gl_mat1" consists of different translations, scalings and a rotation operation.
The following section of code composes the matrix "x_view", which is a major subset of "x_gl_mat1". The matrix "x_gl_mat1" consists of multiple translations, scales and one rotation operation. The individual operation steps are shown in a picture below.


{{Code|Section of axes::properties::update_camera ()"|<syntaxhighlight lang="C" style="font-size:13px">
{{Code|Section of axes::properties::update_camera ()"|<syntaxhighlight lang="C" style="font-size:13px">
   // Unit length vectors for direction of view "f" and up vector "UP"
   // Unit length vector for direction of view "f" and up vector "UP"
   ColumnVector F (c_center), f (F), UP (c_upv);
   ColumnVector F (c_center), f (F), UP (c_upv);
   normalize (f);
   normalize (f);
   normalize (UP);
   normalize (UP);
  // Scale "UP" vector, so that norm(f x UP) becomes 1
  if (std::abs (dot (f, UP)) > 1e-15)
    {
      double fa = 1 / sqrt (1 - f(2)*f(2));
      scale (UP, fa, fa, fa);
    }


   // Calculate the vector rejection UP onto f
   // Calculate the vector rejection UP onto f
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</syntaxhighlight>}}
</syntaxhighlight>}}


To visualize the matrix properties, the "x_gl_mat1" matrix is multiplied by the object coordinates. The plot box is now aligned with the Z-axis and the view point is at the origin <math display="inline">[0,0,0]</math>. The matrix transforms world coordinates into camera coordinates. The purple planes show the near and far clipping planes.
=== x_gl_mat1 ===


[[File:Octave_x_gl_mat1_setup.png|center|250px]]
To visualize the matrix properties, the "x_gl_mat1" matrix is multiplied by the object coordinates. After the transformation, the plot box is aligned with the Z-axis and the view point is at the origin <math display="inline">[0,0,0]</math>. The matrix transforms world coordinates into camera coordinates. The purple planes show the near and far clipping planes.
 
[[File:Octave_x_gl_mat1_setup.png|center|300px]]


The individual translation, scaling and rotation operations of "x_gl_mat1", are shown in the following figure:
The individual translation, scaling and rotation operations of "x_gl_mat1", are shown in the following figure:


[[File:Octave_x_gl_mat1_steps.png|center|1100px]]
[[File:Octave_x_gl_mat1_steps.png|center|1100px]]
== The role of "x_gl_mat2" ==
=== Bounding box ===
The matrix "x_gl_mat2" is composed of the sub matrices "x_viewport" and "x_projection". The purpose of these matrices is to fit the associated 2D image of the above transformation result into a "bounding box". The bounding box is defined as follows:
* bb(0), bb(1): Position of the "viewport"
* bb(2), bb(3): Width and height of the "viewport"
Hint: If you debug in "update_camera ()", you can print "bb": 
  (gdb) print *bb.rep.data@bb.rep.len
  (gdb) $1 = {72.79, 31.50, 434, 342.29}
Compare the result with the output on the Octave prompt:
  hax = axes ();
  get (hax, "position")
  ans = 73.80 47.20 434.00 342.30
  get (gcf, 'position')
  ans = 22 300 560 420
Where 420 - 342.30 - 31.5 + 1 = 47.20
=== x_projection ===
In the following simplified code section the matrix "x_projection" is composed. It is used to normalize the image of the above transformation. For this purpose, the field of view (FOV) must be calculated:
{{Code|Section of axes::properties::update_camera ()"|<syntaxhighlight lang="C" style="font-size:13px">
  if (cameraviewanglemode_is ("auto"))
    {
      if ((bb(2)/bb(3)) > (xM/yM))
        // When the image is scaled to the size of the bounding box,
        // the height collides with the bounding box first. Therefore,
        // the camera view angle is defined by the image height yM.
        af = 1.0 / yM;
      else
        // The image width collides with the bounding box.
        af = 1.0 / xM;
      // The view angle "v_angle", also called field of view "FOV",
      // is formed by the hypotenuse and the adjacent side, which is given by
      // the distance between the view point and the camera target "norm (F)".
      // The ratio of the opposite side, given by "af", to the adjacent side in
      // a right-angled triangle is the tangent of the view angle.
      v_angle = 2 * (180.0 / M_PI) * atan (1 / (2 * af * norm (F)));
      cameraviewangle = v_angle;
    }
  else
    v_angle = get_cameraviewangle ();
  // x_projection: identity "diag([1, 1, 1, 1])"
  Matrix x_projection = xform_matrix ();
  // Calculate backwards from the angle to the ratio. This step
  // is necessary because "v_angle" can be set manually.
  double pf = 1 / (2 * tan ((v_angle / 2) * M_PI / 180.0) * norm (F));
  // Normalize to one. Resulting coordinates are "normalized device coordinates".
  scale (x_projection, pf, pf, 1);
</syntaxhighlight>}}
=== x_viewport ===
"x_viewport" is a transformation used to place the previously "normalized" plot box in the center and to fit it tightly into the bounding box:
{{Code|Section of axes::properties::update_camera ()"|<syntaxhighlight lang="C" style="font-size:13px">
  double pix = 1;
  if (autocam)
    {
      if ((bb(2)/bb(3)) > (xM/yM))
        pix = bb(3);
      else
        pix = bb(2);
    }
  else
    pix = (bb(2) < bb(3) ? bb(2) : bb(3));
  // x_viewport: identity "diag([1, 1, 1, 1])"
  Matrix x_viewport = xform_matrix ();
  // Move to the center of the bounding box inside the figure.
  translate (x_viewport, bb(0)+bb(2)/2, bb(1)+bb(3)/2, 0);
  // Scale either to width or height, to fit correctly into the bounding box
  scale (x_viewport, pix, -pix, 1);
  x_gl_mat2 = x_viewport * x_projection;
</syntaxhighlight>}}
Note: The matrix "x_gl_mat2" scales x, y. However the z-coordinate is not modified!
= setup_opengl_transformation () =
== OpenGL backend ==
In the OpenGL backend, the view matrix, an orthographic matrix, and the viewport transform are used to transform the octave plot into the screen window.
{{Code|Section of opengl_renderer::setup_opengl_transformation ()"|<syntaxhighlight lang="C" style="font-size:13px">
  Matrix x_zlim = props.get_transform_zlim ();
  xZ1 = x_zlim(0)-(x_zlim(1)-x_zlim(0))/2;
  xZ2 = x_zlim(1)+(x_zlim(1)-x_zlim(0))/2;
  // Load x_gl_mat1 and x_gl_mat2
  Matrix x_mat1 = props.get_opengl_matrix_1 ();
  Matrix x_mat2 = props.get_opengl_matrix_2 ();
  m_glfcns.glMatrixMode (GL_MODELVIEW);
  m_glfcns.glLoadIdentity ();
  m_glfcns.glScaled (1, 1, -1);
  // Matrix x_gl_mat1
  m_glfcns.glMultMatrixd (x_mat1.data ());
  m_glfcns.glMatrixMode (GL_PROJECTION);
  m_glfcns.glLoadIdentity ();
  Matrix vp = get_viewport_scaled ();
  // Install orthographic projection matrix with viewport
  // setting "0, vp(2), vp(3), 0" and near / far values "xZ1, xZ2"
  m_glfcns.glOrtho (0, vp(2), vp(3), 0, xZ1, xZ2);
  // Matrix x_gl_mat2
  m_glfcns.glMultMatrixd (x_mat2.data ());
  m_glfcns.glMatrixMode (GL_MODELVIEW);
  m_glfcns.glClear (GL_DEPTH_BUFFER_BIT);
</syntaxhighlight>}}
Hint: If you debug in "setup_opengl_transformation ()", you can print the viewport "vp": 
  (gdb) print *vp.rep.data@vp.rep.len
  (gdb) $1 = {0, 0, 560, 420}
This is consistent with the window size.
Hg200
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